After the introduction of the electromagnetic and circuit theories, along with their numerical solution techniques, it is clear to see that there is a distinct separation between the two. Mathematically, circuit theory is less expensive to solve, it does not need the complicated vector operation, and at a certain frequency range it is suitable for most electronic system designs. The SPICE simulator, its offspring and variations are still the workhorses for most current digital and analog circuit analyses. Models for different circuit components are built around the simulator to accommodate the evolution of semiconductor processing technology.